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Chlorine 36 (36Cl, T1/2 = 301,000 years) is a radionuclide with natural and anthropogenic origin that can be rejected during decommissioning of nuclear power plants or chronically during recycling of used nuclear fuels. Once emitted into the atmosphere, chlorine 36 (gas and particles) can be transferred to the soil and vegetation cover by dry and wet deposition. However, quantitative constraints of these deposits are very scarce. Because of its relatively high mobility in the geosphere and its high bioavailability, chlorine 36 fate in the environment should be studied for environmental and human impact assessments.The aim of this study is therefore to develop dry depositional models for the gaseous and particulate fractions of chlorine 36. The model used for the parameterization of gaseous chlorine 36 dry deposition on grass is the << Big-leaf >> model based on the electrical analogy (Seinfield 1985). It was adapted from the Bah (2020) model developed for iodine. For particulate chlorine 36, the Damay-Pellerin (2017) model was used. This model requiring the diameter of particulate chlorine 36 to be known, a sample of aerosol was taken in the chlorine 36 plume to determine on which particle size it is fixed. The sampling was performed on a low pressure impinger (LPI, DEKATI) of 13 levels for particles with diameters between 30nm and 10μm. In order to obtain model input data, meteorological and micrometeorological parameters were measured continuously at the IRSN La-Hague technical platform (PTILH, France). The PTILH is located 2 km north of Orano La-Hague plant which emits small quantities of chlorine 36. An ultrasonic anemometer (Young 81000V) installed at a height of 4,5 m from the ground measured the direction (°), the velocity u (m/s) and the air friction velocity u* (m/s), the sensible heat flux H, the Monin-Obukhov length (L) and the atmospheric stability (1/L). A weather station (Spectrum Watchdog, series 2000) measured temperature Ts (°C), relative humidity RH (%), dew point (°C), global radiation SR (Wat/m2) and photosynthetic active radiation PAR (μM/m2.s). Sampling campaigns of 2 weeks were also conducted at the PTILH site to determine experimental depositional rates of chlorine 36. Chlorine 36 measurements were carried out by acceleration mass spectrometry at CEREGE-LN2C (France).The particle size distribution of the aerosol sample from chlorine 36 plume shows two peaks, a main one at 2μm and a second one at 10μm, typical distribution of marine aerosol. The models yield chlorine 36 depositional velocities between 6,7.10-3 and 10-2 m/s for the particulate fraction, and between 5.10-3 and 1,3.10-2 m/s for the gaseous one. The total dry depositional velocities (particles and gas) calculated from the model are less than one order of magnitude than the ones obtained experimentally.

Transient topography represents an opportunity for extracting information on the combined effect of tectonics, mantle-driven processes, lithology and climate across different temporal and spatial scales. The geomorphic signature of transient conditions can be used to unravel the landscape evolution and to assess perturbations in uplift rates, especially in areas devoid of stratigraphic constraints. The Atlas-Meseta system experienced a large scale topographic rejuvenation during the Cenozoic through a combination of different processes. Despite the uplift, the Western Moroccan Meseta (WMM) represents a quiescent tectonic domain with deeply incised valleys and high-standing erosional surfaces (relict landscape). This topography is characterized by elevated non-lithological knickpoints, that delimit an uplifted relict landscape, implying a transient response to a change in uplift rates. Here, we determine denudation rates of selected watersheds and bedrock outcrops from cosmogenic nuclides and perform stream profile, regional and basin-scale geomorphic analysis. Denudation rates of the relict and the rejuvenated landscape range from 15 to 20 m/Myr and from 30 to 40 m/Myr, respectively. These results allow estimating the erodibility parameter for performing river-profile inversions and hence extracting rock uplift rates through time. Inverted rock uplift rates are 10-25 m/Myr from 45 to 22 Ma and 30-55 m/Myr from 22 to 10 Ma. Despite the different time scales, the inverted rates are consistent with 10Be averaged denudation rates (15-20 and 30-40 m/Myr) and river incision values from Pleistocene lava flows (

Topographic relief results from the complex interactions between tectonics and erosional surface processes. The efficiency of surface processes is a function of topographic slopes, bedrock erodibility and climatic conditions. Ancient orogens offers a favourable setting to isolate the contribution of lithology, as tectonically driven surface uplift is typically negligible and channel steepness is directly controlled by bedrock erodibility. The Anti-Atlas in NW Africa is a late Paleozoic orogen that contains a well-preserved, uplifted, relict landscape that has been slowly eroding since the late Cretaceous. Here, we combine geomorphic analysis with 10Be-derived denudation rates, to quantify the impact of bedrock erodibility and get insight into the surface evolution of the Anti-Atlas and the adjacent Siroua Massif. Specifically, we show that basin-wide denudation rates from the relict landscape range from 5 to 20 m/Myr, in agreement with the average long-term rates estimated from eroded volumes of Miocene volcanics and available thermochronometric data. This suggests that the regional relict topography has attained an erosional steady state and has been slowly decaying over geological time. Our results are comparable with data from other tectonically quiescent settings and demonstrate a positive linear correlation between denudation rates and normalized channel steepness indices. This allows constraining a narrow range of bedrock erodibility values for different rock-types (quartzite, granitic and sedimentary rocks), that are comparable with estimates from different stable settings. Finally, our compilation from tectonically inactive regions indicates that channel steepness, denudation rates and bedrock erodibility do not change significantly across different climatic zones and precipitation regimes highlighting the critical role of lithology in controlling the topographic relief.

This research has been funded by the Spanish Ministry of Science and Innovation (Projects: CGL2015-66997-R and PID2021-126357NB-100), and by the European Regional Development Fund (FEDER) and Plan de Ciencia Tecnología e Innovación del Principado de Asturias (PCTI) (Grant reference: FC-GRUPIN-IBI/2018/00076/040). The ASTER AMS national facility (CEREGE, Aix-en-Provence) is supported by the INSU/CNRS, the ANR through the “Projets thématiques d’excellence” program for the “Equipements d’excellence” ASTER- CEREGE action and IRD.

Rock avalanches are voluminous debris instabilities originated from a fallen portion of bedrock that suffers pervasive disintegration [1]. Tough rock avalanches are foreseen as low-frequency events, their volume (>106 m3) and runout distance (kilometric scale) make them extremely hazardous processes in populated mountain settings and key contributors to landscape denudation [2]. Deciphering the timing of recurrent rock avalanches is crucial to understand the triggering factors involved in their origin (e.g., role of seismicity) and for risk assessment.This study focuses on a rock avalanche cluster preserved in the southern flank of Sierra de la Sobia; a limestone massif located in the Cantabrian Mountains (North Spain). The rock avalanche cluster analyzed is spatially related to the Marabio Fault trace, which shows unequivocal evidence of Quaternary activity [3]. Rock avalanches in this area have been interpreted as coseismic based on the following evidence [4]: (i) boulder populations show fractal block-size distributions consistently with dynamic fragmentation; (ii) the kinematic analysis of local minor transverse and parallel faults points to a horizontal N-S compression consistent with the regional stress field; and (iii) slope stability analysis indicates that headscarps will turn unstable if ground acceleration peak rises to 0.10–0.15 g during an earthquake, which is within the values expected according to the 2013 European Seismic Hazard Map. A first attempt of numerical dating was performed through the U-Th technique on calcite cements found in the oldest rock avalanches. Results suggest multiple episodes of cementation during the last ~280 ka, but age dispersion hinders the age bracketing of instability events4. Here we present a collection of twenty cosmic ray exposure ages relying on the isotope Cl-36 obtained from limestone boulders sampled in the youngest accumulation bodies of the Entrago and Carrea rock avalanches. Results allow to constrain up to 5 instability events spanning the last ~22 ka and occurring at average recurrence intervals of ~3.6 ka. The youngest rock avalanche event took place 8.5 ka ago and left boulder accumulations close to the headscarp of both rock avalanches. Boulder age dispersion increases accordingly with the increase in runout distance from the headscarp, possibly due to the spatial overlapping of accumulation bodies resulting from different instability events of seismic origin. These preliminary results are promising, because if extended to other rock avalanche clusters of the Cantabrian Mountains, they could help to decipher the recurrence time of severe earthquakes in mountain settings where tectonic deformation occurs at low to moderate rates.[1] Hermanns, R. L. Encyclopedia of Natural Hazards. vol. 2 (2013).[2] Davies, T. Rock Avalanches. In Oxford Research Encyclopedia of Natural Hazard Science 58 (Oxford University Press, 2018). doi:10.1093/acrefore/9780199389407.013.326.[3] Fernández, F. J., Alonso, J. L. & Pando, L. Evidence for quaternary tectonic activity in the western cantabrian Zone (Passes of Marabio, Sobia nappe). Geogaceta 64, 1–3 (2018).[4] Fernández, F. J., Menéndez-Duarte, R., Pando, L., Rodríguez-Rodríguez, L. & Iglesias, M. Gravitational slope processes triggered by past earthquakes on the Western Cantabrian Mountains (Sierra de la Sobia, Northern Spain ). Geomorphology 390, 107867 (2021).

We present a new set of 25 10Be surface exposure ages of boulders located on the Pomeranian moraines and of erratic boulders located directly upstream of the Pomeranian moraines in northern Poland. Together with recalculated 10Be surface exposure ages along the Pomeranian Phase ice-marginal belt from Denmark to the west to Lithuania and Belarus to the east, the full data set (n = 86) enabled us to constrain the timing of the ice front standstill and its subsequent retreat. The investigated area consists of geomorphological record along ~2000 km of the ice margin associated with the ice sheet limit correlated so far with the Late Weichselian Pomeranian Phase (Bælthav in Denmark, Baltija in Lithuania and Braslav in Belarus).We constrained the age of the ice margin position in the area occupied by the Baltic Ice Stream to ~20–19 ka, in the area occupied by the Odra Ice Stream to ~19–18 ka, in the interstream area between the Odra and the Vistula Ice Streams to ~20–19 ka, in the area occupied by the Vistula Ice Stream to ~19–18 ka, in the area occupied by the Mazury Ice Stream to ~18–17 ka, in the area occupied by the Riga Ice Stream to ~17–16 ka, and in the area occupied by the Novgorod Ice Stream to ~16–15 ka. Our best age estimates are based on: (1) a minimum age of the ice margin retreat inferred from new and recalculated 10Be ages of boulders as well as interpretation of available radiocarbon ages from organic deposits and OSL ages from sediments overlying tills, and (2) a maximum age of the ice margin stillstand and retreat inferred from interpretation of available OSL ages from sandur sediments deposited in front of the ice sheet. The asynchrony of the ice margin positions along the Pomeranian Phase ice-marginal belt shows about 3–5 ka difference between the Bælthav ice margin in Denmark and the Braslav ice margin in Belarus. We propose a new scenario of the Pomeranian Phase ice sheet evolution and a time-slice reconstruction of the last Scandinavian Ice Sheet’s southern fringe for the period ~20–15 ka.

International audience; Topographic relief results from the complex interactions between tectonics and erosional surface processes, which are primarily mediated by bedrock erodibility and climatic conditions. Ancient orogens offer a favourable setting to isolate the contribution of lithology, as tectonically driven rock uplift is typically negligible and rock strength variability can exert a critical role on the evolution of the topography. The Anti-Atlas in NW Africa is a late Paleozoic orogen comprising a well-preserved, elevated, relict landscape delimited by non-lithological knickpoints, that was uplifted during a regional late Cenozoic phase of topographic rejuvenation. Here, we combine a geomorphic analysis with 10Be-derived denudation rates to quantify bedrock erodibility and get insight into the surface evolution of the Anti-Atlas and the adjacent Siroua Massif. Specifically, we show that 10Be basin-wide denudation rates for the relict landscape are rather uniform and range from 5 to 12 m/Myr. These rates agree with long-term rates estimated from different methods suggesting that the relict topography archives erosional quasi-erosional steady-state conditions at least since the latest Cretaceous. The uniformly low 10Be denudation rates in the relict topography are consistent despite the variability in channel steepness and topographic relief that correlates with changes in rock type. The expansion of this analysis to the denudation rates of the downstream portion of the landscape, allows to demonstrate a linear relationship between denudation and channel steepness for catchments draining quartz bearing lithologies. This provides the chance to constrain a narrow range of bedrock erodibility values for different rock-types (quartzite, granitic and sedimentary rocks). These values are comparable with estimates from other slowly deforming settings. Specifically, our compilation from tectonically inactive to slow tectonic regions indicates that bedrock erodibility does not change significantly across different climatic zones and precipitation regimes. This highlights the critical role of lithology in controlling the production of topographic relief in post-orogenic/slow tectonic settings. Finally, we calculate the predicted denudation rates for the steeper portions of the landscape that adjusted to the new uplift rates based on the linear correlation between erosion rates and normalized steepness indices. These rates range from 20 to 50 m/Myr and agree with the direct measurements from two catchments.